1. Field of the Invention
The present invention relates to a liquid-crystal display device and to an aligning agent for liquid crystals.
2. Description of the Background
Twisted nematic (TN) and dynamic scatter (DS) types are the modes of liquid-crystal display elements extensively used at present. Because these liquid-crystal display modes have the disadvantages of small visual angle and low contrast, supertwisted nematic (STN) and thin-film transistor (TFT) types have appeared on the market in recent years as new display modes. These new modes all employ nematic liquid-crystal cells, and hence it is inevitable that the response speed is on the order of milliseconds. In order to solve this problem, studies are actively under way on the display mode using ferroelectric liquid crystals whose response speed is as high as on the order of microseconds. The system of thermal writing in smectic A phase is also known, which features high-precision display.
In these liquid-crystal display devices, the molecules of liquid crystals involved are oriented in a specific direction, and display is effected by applying a voltage upon the oriented molecules to change the orientation. A thin film (called the aligning layer) is formed on each electrode to orient the molecules of liquid crystals in the display device in a specific direction. The material of aligning agent for liquid crystals comprised in the aligning layer is of two types: one is an organic polymer, and the other is an obliquely deposited inorganic oxide. The mechanism of the aligning agent or orientator comprising an organic polymer is described by J. M. Geary, et al. (Journal of Applied Physics, 15, Nov., 1987). In the latter case, the inorganic oxide is deposited onto the electrode in a specific direction to form a small column made of the oxide on the electrode, thereby orienting the liquid crystal molecules along this column. Polyimides and other organic polymers are widely used as aligning agents made of the organic polymer, while silicon dioxide and other inorganic oxides are mainly studied as that made of the inorganic oxide.
Polyimides and other organic polymers are now widely used as the aligning agent in nematic liquid-crystal cells. A polymer is dissolved in a common solvent at a proper concentration, the solution thus obtained is applied on the electrode by spin coating or dipping to form a thin film tens to thousands .ANG. thick, and the formed film is rubbed with a sheet of cloth or the like to become oriented along a specific axis, thus giving a large area of aligning layer rather easily. Of the organic polymers, polyimides have been extensively studied as an aligning agent in ferroelectric liquid-crystal display devices. However, when polyamides are used, it is not possible to obtain high contrast. Further, defects tend to form and memory is hard to reproduce because the liquid crystal molecules in the display device are subject to twist or splay orientation in contrast to the orientation direction indicated by the aligning layer (the axial direction of the molecules gradually deviates from the indicated direction as the molecules approach the inside of the cell from the electrode).
In an effort to overcome these disadvantages, studies have been made on orientation by oblique deposition of an inorganic oxide, but this method is not amenable to industrialization because it is difficult to produce cells of large area. For these reasons, there has been a demand for the development of an aligning agent for liquid crystals made of an organic polymer with excellent characteristics. The display mode of the supertwisted nematic type has also demanded an aligning agent for liquid crystals which will provide display devices of little display unevenness, high contrast and low dependence on visual angle. In ferroelectric liquid-crystal cells, in particular, memory characteristics, high contrast, dependence on visual angle and other important characteristics are dictated by the aligning layer used. Hence, there has been a demand for the development of an excellent aligning agent for liquid crystals in both the nematic and smectic type display modes.
As an example of organic polymers employed as an aligning agent for liquid crystals, is known an optically active polyamino acid (disclosed in Japanese Patent Publication No. 35780/1979). Since the formed aligning layer itself is optically active in this case, the director of liquid crystal molecules tends to tilt from the direction indicated by the aligning direction in nematic liquid-crystal display devices; and when this type of polymer is used in ferroelectric liquid-crystal cells, high contrast and stable memory characteristics cannot be achieved, because the layered structure of liquid crystal molecules tends to tilt from the direction indicated by the aligning agent and uniform orientation is hardly ensured. In addition, optically active polyamino acids, which are likely to assume an .alpha.-helical structure, are sparingly soluble in common solvents, and hence a special solvent, such as trifluoroacetic acid, has to be used.
Hence, a need continues to exist for polymers which can act as an aligning agent for liquid crystals, and which provide good memory characteristics and uniform orientation to ensure high contrast in ferroelectric liquid-crystal cells.